Port fuel injection is a popular approach to preparation of an air-fuel mixture for combustion in an automotive engine. A port fuel injection system commonly has a central air inlet, from which air is distributed through an inlet manifold to the intake ports of the engine combustion chambers, and fuel injectors which deliver fuel into the inlet manifold adjacent each intake port. A throttle in the central air inlet controls the air flow to the engine, and the amount of fuel delivered by the injectors is proportioned to the air flow.
In such a system, the pressure in the inlet manifold varies as the operator moves the throttle in response to changing requirements for engine speed and load. If the fuel injectors deliver fuel directly into the inlet manifold, variations in the inlet manifold pressure will affect fuel delivery, and the system must have provision to compensate for manifold pressure variations. For example, in one port fuel injection system having electromagnetically operated fuel injectors, the injectors deliver fuel according to a predetermined duty cycle, and it is desired that the amount of fuel delivered by the injectors respond only to variations in the duty cycle. To avoid having variations in the inlet manifold pressure affect fuel delivery, the pressure of the fuel supplied to the injectors may be controlled so that the difference between the fuel supply pressure and the inlet manifold pressure is constant. However, to allow the fuel supply pressure to follow the wide variations in inlet manifold pressure, the fuel supply pump must be capable of supplying fuel over the corresponding range of fuel supply pressures.
In another port fuel injection system, the effect of inlet manifold pressure on injector fuel delivery is avoided by providing the injectors with an atmospheric vent. In such an arrangement, mixture nozzles open into the inlet manifold from a region of atmospheric pressure, and each injector delivers fuel across the atmospheric pressure region and through a mixture nozzle into the inlet manifold. The injectors thus discharge into a region of substantially constant atmospheric pressure, and the fuel supply system may be adapted to supply fuel at a substantially constant pressure.
However, to control air flow in such a system, air flow through the mixture nozzles is limited by a sonic flow restriction in each nozzle; air flow through the nozzles is thus restricted at the low manifold pressures which occur during low air flow conditions. As a result, when the manifold pressure increases in response to an increase in air flow to the engine, air flow through the mixture nozzles decreases; thus the increased fuel flow required by the increased air flow to the engine is accompanied by a decrease in air flow through the mixture nozzles, inhibiting the air-fuel mixing process.